Imaging element for making an improved printing plate according to the silver salt diffusion transfer process

ABSTRACT

The present invention provides an imaging element comprising in the order given on a hydrophilic base (i) an image receiving layer containing physical development nuclei, (ii) a photosensitive layer containing a silver halide emulsion being in water permeable relationship with said image receiving layer and (iii) an antistress layer being in water permeable relationship with said image receiving layer and comprising gelatin, characterized in that said antistress layer comprises at least 30 mg/m 2  of a matting agent with a weight average diameter of at least 3 μm.

Benefit is claimed under 35 U.S.C. 119(e) from provisional application Ser. No. 60/026,857 filed Sep. 30, 1996.

1. FIELD OF THE INVENTION

The present invention relates to an imaging element, for making improved lithographic printing plates according to the silver salt diffusion transfer process.

The present invention further relates to a method for making improved lithographic printing plates according to the silver salt diffusion transfer process.

2. BACKGROUND OF THE INVENTION

The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972).

In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.

A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellent ink-receptive areas on a water-receptive ink-repellent background.

The DTR-image can be formed in the image receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR element) or in the image receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image receiving layer in water permeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.

Two types of the mono-sheet DTR offset printing plate exist. According to a first type disclosed in e.g. U.S. Pat. No. 4,722,535 and GB-1,241,661 a support is provided in the order given with a silver halide emulsion layer and a layer containing physical development nuclei serving as the image-receiving layer. After information-wise exposure and development the imaged element is used as a printing plate without the removal of the emulsion layer.

According to a second type of mono-sheet DTR offset printing plate a hydrophilic base is provided in the order given with a layer of physical development nuclei and a silver halide emulsion layer. After information-wise exposure and development the imaged element is treated to remove the emulsion layer so that a hydrophilic base carrying a silver image is left wich is used as a printing plate. Such type of lithographic printing plate is disclosed e.g. in U.S. Pat. No. 3,511,656.

As for other printing plates it is required that the offset printing plates belonging to the second type of mono-sheet DTR offset printing plates have good printing properties e.g. a high printing endurance, good ink acceptance in the printing areas and no ink acceptance in the non-printing areas (no staining). It is furthermore required that the printing plate exhibits a high resolving power and good physical properties.

Offset printing plates belonging to the second type of mono-sheet DTR offset printing plates have in most respects good printing properties. However, their printing plate precursors exhibit poor physical properties. In order to avoid an unwanted increase in photographic sensitivity when storing the printing plate precursors of said printing plate these printing plate precursors have to be packed with paper interleaves what is a supplemental cost and causes problems in the automatic processing of said printing plate precursors. Furthermore the handling of the unprocessed material results in fingerprint marks on said material which marks show up on the processed material as places with a reduced or zero silver yield and thus a reduced or zero ink acceptance on these places.

It would be greatly appreciated if these problems would be solved.

3. SUMMARY OF THE INVENTION

It is an object of the present invention to provide an imaging element for making offset printing plates according to the silver salt diffusion transfer process which is not sensitive to fingerprints.

It is a further object of the present invention to provide an imaging element for making offset printing plates according to the silver salt diffusion transfer process which can be packed without paper interleaves without marked sensitivity changes when storing said imaging element.

It is a further object of the present invention to provide a method for making offset printing plates with said imaging element having the above mentioned advantage.

Further objects of the present invention will become clear from the description hereinafter.

According to the present invention there is provided an imaging element comprising in the order given on a hydrophilic base (i) an image receiving layer containing physical development nuclei, (ii) a photosensitive layer containing a silver halide emulsion being in water permeable relationship with said image receiving layer and (iii) an antistress layer being in water permeable relationship with said image receiving layer and comprising gelatin, characterized in that said antistress layer comprises at least 30 mg/m² of a matting agent with a weight average diameter of at least 3 μm.

According to the present invention there is also provided a method for making an offset printing plate according to the silver salt diffusion transfer process comprising the steps of:

(a) image-wise exposing an imaging element according to the present invention,

(b) applying an aqueous alkaline solution to the imaging element in the presence of (a) developing agent(s) and (a) silver halide solvent(s),

(c) treating the imaging element to remove the layer(s) on top of said image receiving layer, thereby uncovering said silver image formed in said image receiving layer.

4. DETAILED DESCRIPTION OF THE INVENTION

Lithographic printing plates are obtained according to the DTR-process having good printing properties from imaging elements having good physical properties by using an imaging element comprising an upper layer (antistress layer) comprising gelatin when said antistress layer comprises at least 30 mg/m² of a matting agent with a weight average diameter of at least 3 μm.

According to the invention said matting agent may be a water-insoluble, alkali soluble organic polymer or copolymer such as a copolymer of acrylic acid and methyl acrylate. More preferably said matting agent is a water insoluble inorganic compound such as silica. Most preferably said matting agent is an alkali-insoluble organic polymer or copolymer such as a polymer or copolymer of methyl (meth)acrylate. A particularly preferred matting agent is a copolymer of styrene, methyl methacrylate and maleic acid.

According to the invention the matting agent has a weight average diameter of at least 3 μm, more preferably of at least 4 μm, most preferably of at least 5 μm. The maximum weight average diameter is not so important but is for practical reasons preferably less than 15 μm, more preferably less than 10 μm.

According to the invention said antistress layer comprises at least 30 mg/m² of a matting agent with a weight average diameter of at least 3 μm, more preferably between 50 and 500 mg/m² of said matting agent, most preferably between 100 and 200 mg/m² of said matting agent.

According to the invention the antistress layer comprises preferably unhardened gelatin or an unhardened gelatin admixture preferably in an amount between 0.2 and 2 g/m², more preferably in an amount between 0.4 and 1.75 g/m², most preferably in an amount between 0.6 and 1.25 g/m².

Preferably at least 50%, more preferably at least 75%, most preferably at least 90% by weight of said unhardened gelatin or gelatin admixture belongs to one or more gelatin species whereof a 10% by weight aqueous solution at 36° C. and pH 6 has a viscosity lower than 35 mPa.s, more preferably lower than 30 mPa.s at a shearing rate of 1000 s⁻¹.

Said antistress layer can comprise more than one species of unhardened gelatin whereof a 10% by weight aqueous solution at 36° C. and pH 6 has a viscosity lower than 35 mPa.s at a shearing rate of 1000 s⁻¹, but it is preferred for practical reasons that said layer comprise only one such gelatin.

Said antistress layers may contain small particles e.g matting agents with a mean diameter between 0.2 and 3 μm in order to improve the diffusion of processing solutions through said antistress layers.

The antistress layer is the surface layer of the imaging element lying on that side of the support that carries the emulsion layer.

The antistress layer according to the invention has to be in water permeable relationship with said image receiving layer in the imaging element. Layers being in water permeable contact with each other are layers that are contiguous to each other or only separated from each other by (a) water permeable layer(s). The nature of a water permeable layer is such that it does not substantially inhibit or restrain the diffusion of water or of compounds contained in an aqueous solution e.g. developing agents or the complexed silver.

The imaging element is preferably prepared by coating the different layers on a hydrophilic base. Alternatively the different layers may be laminated to said image receiving layer from a temporary base holding the layers in reverse order as disclosed in U.S. Pat. No. 5,068,165.

Said hydrophilic base is a hardened hydrophilic layer, containing a hydrophilic binder and a hardening agent coated on a flexible support.

Such hydrophilic binders are disclosed in e.g. EP-A 450,199, which therefor is incorporated herein by reference. Preferred hardened hydrophilic layers comprise partially modified dextrans or pullulan hardened with an aldehyde as disclosed in e.g. EP-A 514,990 which therefor is incorporated herein by reference. More preferred hydrophilic layers are layers of polyvinyl alcohol hardened with a tetraalkyl orthosilicate and preferably containing SiO₂ and/or TiO₂ wherein the weight ratio between said polyvinylalcohol and said tetraalkyl orthosilicate is between 0.5 and 5 as disclosed in e.g. GB-P 1,419,512, FR-P 2,300,354, U.S. Pat. No. 3,971,660, U.S. Pat. No. 4,284,705, EP-A 405,016 and EP-A 450,199 which therefor are incorporated herein by reference.

Flexible supports may be opaque or transparent, e.g. a paper support or resin support. When a paper support is used preference is given to one coated at one or both sides with an Alpha-olefin polymer. It is also possible to use an organic resin support e.g. poly(ethylene terephthalate) film or poly-Alpha-olefin films. The thickness of such organic resin film is preferably comprised between 0.07 and 0.35 mm. These organic resin supports are preferably coated with a hydrophilic adhesion layer which can contain water insoluble particles such as silica or titanium dioxide.

Subsequent to the preparation of the hydrophilic base of a support as described above, said hydrophilic base may be immediately coated with a solution containing the physical development nuclei or may be coated with said solution at a later stage.

The image receiving layer preferably comprises physical development nuclei in an amount ranging from 0.1 mg to 20 mg/m². The image receiving layer containing physical development nuclei may be free of hydrophilic binder but preferably comprises small amounts upto 80% by weight of the total weight of said layer of a hydrophilic colloid e.g. polyvinyl alcohol to improve the hydrophilicity of the surface.

Preferred development nuclei for use in accordance with the present invention are sulphides of heavy metals e.g. sulphides of antimony, bismuth, cadmium, cobalt, lead, nickel, palladium, platinum, silver, and zinc. Especially suitable development nuclei in connection with the present invention are palladium sulphide nuclei. Other suitable development nuclei are salts such as e.g. selenides, polyselenides, polysulphides, mercaptans, and tin (II) halides. Heavy metals, preferably silver, gold, platinum, palladium, and mercury can be used in colloidal form.

The photosensitive layer used according to the present invention may be any layer comprising a hydrophilic colloid binder and at least one silver halide emulsion, at least one of the silver halide emulsions being photosensitive.

The photographic silver halide emulsion(s) used in accordance with the present invention can be prepared from soluble silver salts and soluble halides according to different methods as described e.g. by P. Glafkides in "Chimie et Physique Photographique", Paul Montel, Paris (1967), by G. F. Duffin in "Photographic Emulsion Chemistry", The Focal Press, London (1966), and by V. L. Zelikman et al in "Making and Coating Photographic Emulsion", The Focal Press, London (1966).

For use according to the present invention the silver halide emulsion or emulsions preferably consist principally of silver chloride while a fraction of silver bromide may be present ranging from 1 mole % to 40 mole %. Most preferably a silver halide emulsion containing at least 70 mole % of silver chloride is used.

The average size of the silver halide grains may range from 0.10 to 0.70 μm, preferably from 0.25 to 0.45 μm.

Preferably during the precipitation stage iridium and/or rhodium containing compounds or a mixture of both are added. The concentration of these added compounds ranges from 10⁻⁸ to 10⁻³ mole per mole of AgNO₃, preferably between 10⁻⁷ and 10⁻⁵ mole per mole of AgNO₃.

The emulsions can be chemically sensitized e.g. by adding sulphur-containing compounds during the chemical ripening stage e.g. allyl isothiocyanate, allyl thiourea, and sodium thiosulphate. Also reducing agents e.g. the tin compounds described in BE-P 493,464 and 568,687, and polyamines such as diethylene triamine or derivatives of aminomethane-sulphonic acid can be used as chemical sensitizers. Other suitable chemical sensitizers are noble metals and noble metal compounds such as gold, platinum, palladium, iridium, ruthenium and rhodium. This method of chemical sensitization has been described in the article of R. KOSLOWSKY, Z. Wiss. Photogr. Photophys. Photochem. 46, 65-72 (1951).

Apart from negative-working silver halide emulsions that are preferred for their high photosensitivity, use can be made also of direct-positive silver halide emulsions that produce a positive silver image in the emulsion layer(s) and a negative image on the image-receiving layer.

Suitable direct positive silver halide emulsions for use in accordance with the present invention are silver halide emulsions that have been previously fogged or that mainly form an internal latent image.

The silver halide emulsions of the DTR-element can be spectrally sensitized according to the spectral emission of the exposure source for which the DTR element is designed.

Suitable sensitizing dyes for the visible spectral region include methine dyes such as those described by F. M. Hamer in "The Cyanine Dyes and Related Compounds", 1964, John Wiley & Sons. Dyes that can be used for this purpose include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, homopolar cyanine dyes, hemicyanine dyes, styryl dyes and hemioxonol dyes. Particularly valuable dyes are those belonging to the cyanine dyes, merocyanine dyes, complex merocyanine dyes.

In the case of a conventional light source, e.g. tungsten light, a green sensitizing dye is needed. In case of exposure by an argon ion laser a blue sensizing dye is incorporated. In case of exposure by a red light emitting source, e.g. a LED or a HeNe laser a red sensitizing dye is used. In case of exposure by a semiconductor laser special spectral sensitizing dyes suited for the near infra-red are required. Suitable infra-red sensitizing dyes are disclosed in i.a. U.S. Pat. Nos. 2,095,854, 2,095,856, 2,955,939, 3,482,978, 3,552,974, 3,573,921, 3,582,344, 3,623,881 and 3,695,888.

A preferred blue sensitizing dye, green sensitizing dye, red sensitizing dye and infra-red sensitizing dye in connection with the present invention are described in EP-A 554,585.

To enhance the sensitivity in the red or near infra-red region use can be made of so-called supersensitizers in combination with red or infra-red sensitizing dyes. Suitable supersensitizers are described in Research Disclosure Vol 289, May 1988, item 28952. The spectral sensitizers can be added to the photographic emulsions in the form of an aqueous solution, a solution in an organic solvent or in the form of a dispersion.

The silver halide emulsions may contain the usual emulsion stabilizers. Suitable emulsion stabilizers are azaindenes, preferably tetra- or penta-azaindenes, especially those substituted with hydroxy or amino groups. Compounds of this kind have been described by BIRR in Z. Wiss. Photogr. Photophys. Photochem. 47, 2-27 (1952). Other suitable emulsion stabilizers are i.a. heterocyclic mercapto compounds.

As binder in the silver halide emulsion layer(s) in connection with the present invention a hydrophilic colloid may be used, usually a protein, preferably gelatin. Gelatin can, however, be replaced in part or integrally by synthetic, semi-synthetic, or natural polymers. Preferably the silver halide emulsion layer contains at least one gelatin species whereof a 10% by weight aqueous solution at 36° C. and pH 6 has a viscosity lower than 20 mPa.s at a shearing rate of 1000 s⁻¹ combined with a gelatin of a higher viscosity. The weight ratio of said low viscosity gelatin versus the gelatin of a higher viscosity is preferably >0.5.

Preferably the gelatin layer(s) is(are) unhardened. Unhardened means that when such gelatin layer is coated on a subbed polyethylene terephtalate film base at a dry thickness of 1.2 g/m², dried for 3 days at 57° C. and 35% R.H. and dipped in water of 30° C., said gelatin layer is dissolved for more than 95% by weight within 5 minutes.

The silver halide emulsions may contain pH controlling ingredients. Preferably at least one gelatin containing layer is coated at a pH value not below the iso-electric point of the gelatin. More preferably the emulsion layer is coated at a pH value not below the iso-electric point of the gelatin. Most preferably all the gelatin containing layers are coated at a pH value not below the iso-electric point of their gelatin. Other ingredients such as antifogging agents, development accelerators, wetting agents, and hardening agents for gelatin may be present. The silver halide emulsion layer may comprise light-screening dyes that absorb scattering light and thus promote the image sharpness. Suitable light-absorbing dyes are described in i.a. U.S. Pat. No. 4,092,168, U.S. Pat. No. 4,311,787 and DE-P 2,453,217.

More details about the composition, preparation and coating of silver halide emulsions suitable for use in accordance with the present invention can be found in e.g. Product Licensing Index, Vol. 92, December 1971, publication 9232, p. 107-109.

An intermediate layer, which may be present between said silver halide emulsion containing layer and said image receiving layer may incorporate one or more ingredients such as i.a. antihalation dyes or pigment, developing agents, silver halide solvents, base precursors, and anticorrosion substances.

According to the present invention the imaging element can be information-wise exposed in an apparatus according to its particular application. A wide choice of cameras for exposing the photosensitive silver halide emulsion exists on the market. Horizontal, vertical and darkroom type cameras and contact-exposure apparatus are available to suit any particular class of reprographic work. The imaging element in accordance with the present invention can also be exposed with the aid of i.a. laser recorders and cathode rays tubes.

According to the present invention the development and diffusion transfer of the information-wise exposed imaging element in order to form a silver image in said photosensitive layer and to allow unreduced silver halide or complexes formed thereof to diffuse image-wise from the photosensitive layer to said image receiving layer to produce therein a silver image, are effected with the aid of an aqueous alkaline solution in the presence of (a) developing agent(s), and (a) silver halide solvent(s). The developing agent(s) and/or the silver halide solvent(s) can be incorporated in the aqueous alkaline solution and/or in the maging element.

Preferably a silver halide solvent in the aqueous alkaline solution is used in an amount between 0.05% by weight and 5% by weight and more preferably between 0.5% by weight and 2% by weight. The silver halide solvent, which acts as a complexing agent for silver halide, preferably is a water-soluble thiosulphate or thiocyanate e.g. sodium, potassium, or ammonium thiosulphate and sodium, potassium, or ammonium thiocyanate.

Further silver halide solvents that can be used in connection with the present invention are e.g. sulphite, amines, 2-mercaptobenzoic acid, cyclic imide compounds such as e.g. uracil, 5,5-dialkylhydantoins, alkyl sulfones and oxazolidones.

Further silver halide solvents for use in connection with the present invention are alkanolamines. Examples of alkanolamines that may be used in connection with the present invention correspond to the following formula: ##STR1## wherein X and X' independently represent hydrogen, a hydroxyl group or an amino group, l and m represent 0 or integers of 1 or more and n represents an integer of 1 or more. Said alkanolamines may be present in the alkaline processing liquid in a concentration preferably between 0.1% and 5% by weight. However part or all of the alkanolamine can be present in one or more layers of the imaging element.

Still other preferred further silver halide solvents for use in connection with the present invention are thioethers. Preferably used thioethers correspond to the following general formula:

    Z--(R.sup.1 --S).sub.t --R.sup.2 --S--R.sup.3 --Y

wherein Z and Y each independently represents hydrogen, an alkyl group, an amino group, an ammonium group, a hydroxyl, a sulfo group, a carboxyl, an aminocarbonyl or an aminosulfonyl, R¹, R² and R³ each independently represents an alkylene that may be substituted and optionally contain an oxygen bridge and t represents an integer from 0 to 10. Examples of thioether compounds corresponding to the above formula are disclosed in e.g. U.S. Pat. Nos. 4.960.683 and EP-A 554,585.

Still further suitable silver halide solvents are 1,2,4-triazolium-3-thiolates, preferably 1,2,4-triazolium-3-thiolates substituted with at least one substituent selected from the group consisting of a C₁ -C₈ alkyl group that contains at least 3 fluorine atoms, a C₄ -C₁₀ hydrocarbon group and a 4-amino group substituted with a C₁ -C₈ alkyl group that contains at least 3 fluorine atoms and/or a C₄ -C₁₀ hydrocarbon group.

Combinations of different silver halide solvents can be used and it is also possible to incorporate at least one silver halide solvent into a suitable layer of the imaging element and to add at least one other silver halide solvent to the developing solution.

The alkaline processing liquid may also contain (a) developing agent(s). In this case the alkaline processing liquid is called a developer. On the other hand some or all of the developing agent(s) may be present in one or more layers of the photographic material or imaging element. When all of the developing agents are contained in the imaging element the alkaline processing liquid is called an activator or activating liquid.

Silver halide developing agents for use in accordance with the present invention are preferably of the p-dihydroxybenzene type, e.g. hydroquinone, methylhydroquinone or chlorohydroquinone, preferably in combination with an auxiliary developing agent being a 1-phenyl-3-pyrazolidone-type developing agent and/or p-monomethylaminoplienol. Particularly useful auxiliary developing agents are the 1-phenyl-3-pyrazolidones. Even more preferred, particularly when they are incorporated into the photographic material are 1-phenyl-3-pyrazolidones of which the aqueous solubility is increased by a hydrophilic substituent such as e.g. hydroxy, amino, carboxylic acid group, sulphonic acid group etc.. Examples of 1-phenyl-3-pyrazolidones substituted with one or more hydrophilic groups are e.g. 1-phenyl-4,4-dimethyl-2-hydroxy-3-pyrazolidone, 1-(4-carboxyphenyl)-4,4-dimethyl-3-pyrazolidone etc.. However other developing agents can be used.

Preferred amounts of the hydroquinone-type developing agents are in the range of 0.05 mole to 0.40 mole per liter and preferred amounts of secondary developing agent(s) in the range of 1.8×10⁻³ to 2.0×10⁻¹ mole per liter.

The aqueous alkaline solution in accordance with the present invention may further comprise sulphite e.g. sodium sulphite in an amount ranging from 40 g to 180 g per liter, preferably from 60 to 160 g per liter in combination with another silver halide solvent.

The quantitative ranges given for the developing agents, silver halide solvents, and sulphite apply to the amount of these compounds present as solutes in the aqueous alkaline solution during the DTR-processing, whether these compounds make part of the aqueous alkaline solution or were dissolved from the layers containing them upon application thereto of the aqueous alkaline solution.

The aqueous alkaline solution suitable for use according to the present invention preferably comprises aluminum ions in an amount of at least 0.3 g/l, more preferably in an amount of at least 0.6 g/l in order to prevent sticking of the antistress layer to the transporting rollers when said layer is swollen with the aqueous alkaline solution.

The alkaline processing liquid preferably has a pH between 9 and 14 and more preferably between 10 and 13, but depends on the type of silver halide emulsion material to be developed, intended development time, and processing temperature.

The processing conditions such as temperature and time may vary within broad ranges provided the mechanical strength of the materials to be processed is not adversely influenced and no decomposition takes place.

The pH of the alkaline processing liquid may be established by an organic or inorganic alkaline substance or a combination thereof. Suitable inorganic alkaline substances are e.g. hydroxides of sodium and potassium, alkali metal salts of phosphoric acid and/or silicic acid e.g. trisodium phosphate, orthosilicates, metasilicates, hydrodisilicates of sodium or potassium, and sodium carbonate etc.. Suitable organic alkaline substances are e.g. alkanolamines. In the latter case the alkanolamines will provide or help providing the pH and serve as a silver halide complexing agent.

The aqueous alkaline solution may further comprise hydrophobizing agents for improving the hydrophobicity of the silver image obtained in the image receiving layer. Generally these compounds contain a mercapto group or thiolate group and one or more hydrophobic substituents. Particularly preferred hydrophobizing agents are mercapto-1,3,4-thiadiazoles as described in DE-A 1,228,927 and in U.S. Pat. No. 4,563,410, 2-mercapto-5-heptyl-oxa-3,4-diazole and long chain (at least 5 carbon atoms) alkyl substituted mercaptotetrazoles. The hydrophobizing agents can be used alone or in combination with each other.

These hydrophobizing compounds can be added to the aqueous alkaline solution in an amount of preferably 0.1 to 3 g per liter and preferably in admixture with 1-phenyl-5-mercaptotetrazole, the latter compound may be used in amounts of e.g. 50 mg to 1.2 g per liter of solution, which may contain a minor amount of ethanol to improve the dissolution of said compounds.

The aqueous alkaline solution may comprise other ingredients such as e.g. oxidation preservatives, calcium-sequestering compounds, anti-sludge agents, and hardeners including latent hardeners.

Regeneration of the aqueous alkaline solution according to known methods is, of course, possible, whether the solution incorporates developing agent(s) and/or silver halide solvent(s) or not.

The development may be stopped--though this is often not necessary--with a so-called stabilization liquid, which actually is an acidic stop-bath having a pH preferably in the range from 5 to 7.

Bufferred stop bath compositions comprising a mixture of sodium dihydrogen orthophosphate and disodium hydrogen orthophosphate and having a pH in said range are preferred.

The development and diffusion transfer can be initiated in different ways e.g. by rubbing with a roller, by wiping with an absorbent means e.g. with a plug of cotton or sponge, or by dipping the material to be treated in the liquid composition. Preferably, they proceed in an automatically operated apparatus. They are normally carried out at a temperature in the range of 18° C. to 30° C. and in a time from 5 s to 5 min.

After formation of the silver image on the hydrophilic base an excess of aqueous alkaline solution still present on the base may be eliminated, preferably by guiding the foil through a pair of squeezing rollers.

The silver image thus obtained in the layer of physical development nuclei is subsequently uncovered by treating the imaging element to remove all the layers above the layer containing physical development nuclei, thereby exposing the imaged surface of the hydrophilic base.

According to a particularly preferred embodiment of the present invention the silver image in the layer of physical development nuclei is uncovered by washing off all the layers above the layer containing physical development nuclei with rinsing water.

The temperature of the rinsing water may be varied widely but is preferably between 30° C. and 50° C., more preferably between 35° C. and 45° C.

The imaged surface of the hydrophilic base can be subjected to a chemical treatment that increases the hydrophilicity of the non-silver image parts and the oleophilicity of the silver image.

This chemical after-treatment is preferably carried out with a lithographic composition often called finisher comprising at least one compound enhancing the ink-receptivity and/or lacquer-receptivity of the silver image and at least one compound that improves the ink-repelling characteristics of the hydrophilic base.

Suitable ingredients for the finisher are e.g. organic compounds containing a mercapto group such as the hydrophobizing compounds referred to hereinbefore for the alkaline solution. Preferred compounds correspond to one of the following formulas: ##STR2## wherein R⁵ represents hydrogen or an acyl group, R⁴ represents alkyl, aryl or aralkyl. Most preferably used compounds are compounds according to one of the above formulas wherein R⁴ represents an alkyl containing 3 to 16 C-atoms. Said (a) hydrophobizing agent(s) is(are) comprised in the finisher preferably in a total concentration between 0.1 g/l and 10 g/l, more preferably in a total concentration between 0.3 g/l and 3 g/l.

Additives improving the oleophilic ink-repellency of the hydrophilic base areas are e.g. carbohydrates such as acidic polysaccharides like gum arabic, carboxymethylcellulose, sodium alginate, propylene glycol ester of alginic acid, hydroxyethyl starch, dextrin, hydroxyethylcellulose, polyvinyl pyrrolidone, polystyrene sulphonic acid, polyglycols being the reaction products of ethyleneoxide and/or propyleneoxide with water or an alcohol and polyvinyl alcohol. Optionally, hygroscopic substances e.g. sorbitol, glycerol, tri(hydroxyethyl)ester of glycerol, and turkish red oil may be added.

Furthermore (a) surface-active compound(s) is preferably also added to the finisher. The concentration thereof may vary within broad ranges provided the finisher shows no excessive degree of foaming when plates are finished. Preferred surface-active compound are anionic or non-ionic surface-active compound.

A suitable finisher as disclosed in U.S. Pat. No. 4.563.410 is a composition comprising a solution of a mercaptotriazole in a solution of polyethylene oxide with a molecular weight of 4,000. Further suitable finishers have been described in i.a. U.S. Pat. No. 4.062.682 and EP-A 681219.

At the moment the treatment with the finisher is started the surface carrying the silver pattern may be in dry or wet state. In general, the treatment with the finisher does not take long, usually not longer than about 30 seconds and it may be carried out immediately after the processing and uncovering steps, preferably at a temperature of the finisher in the range from 30° C. to 60° C.

The finisher can be applied in different ways such as by rubbing with a roller, by wiping with an absorbent means e.g. with a plug of cotton or sponge, or by dipping the material to be treated in the finisher. The image-hydrophobizing step of the printing plate may also proceed automatically by conducting the printing plate through a device having a narrow channel filled with the finisher and conveying the printing plate at the end of the channel between two squeezing rollers removing the excess of liquid.

As soon as the hydrophilic base carrying the silver image has been treated with the finisher, it is ready to be used as a printing plate.

The following example illustrates the present invention without however, limiting it thereto. All parts, percentages and ratios are by weight unless otherwise indicated.

EXAMPLE 1

Preparation of the hydrophilic layer coating solution

To 440 g of a dispersion containing 2.1% SiO₂ (average particle size 7 nm) and 2.5% polyvinyl alcohol in deionized water were subsequently added, while stirring, 250 g of a 5% polyvinyl alcohol solution in water, 105 g of a hydrolyzed 22% tetramethyl orthosilicate emulsion in water and 22 g of a 10% solution of a wetting agent. To this mixture was then added 183 g of deionized water and the pH was adjusted to pH=4.

Preparation of the imaging element

The hydrophilic coating solution was coated to a polyethylene terephthalate support provided with a hydrophilic subbing layer such that the SiO₂ coverage was 0.84 g/m² and the polyvinyl alcohol content was 1 g/m².

The thus obtained element was dried and subjected to a temperature of 40° C. for 5 days and then the hydrophilic layer was overcoated with a layer containing 3.1 mg/m² PdS as physical development nuclei and then with a substantially unhardened photosensitive orthochromatically sensitized negative-working cadmium-free gelatin silver chloroiodide emulsion layer (99.8/0.2 mol %) containing 1 mmole/mole AgX of 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene and 2.2 mole/mole AgX of 1-(3-(2-sulphobenzamido))phenyl-5-mercapto-tetrazole, the silver halide being provided in an amount corresponding to 2.50 g of silver nitrate per m² and the gelatin content of the emulsion layer being 1.50 g/m², consisting of 0.7 g/m² of a gelatin with a viscosity of 21 mPas and the remainder of a gelatin with a viscosity of 14 mPas.

Finally the photosensitive emulsion layer was overcoated with an antistress layer containing no hardeners comprising 0.70 g/m² gelatin with a viscosity of 10-12 mPas (gelatin K 7598 of Koepff), and 60 mg/m² of Levanyl Rot (trade name of Bayer A. G. for a red pigment). Imaging element I was so obtained.

Imaging element II was obtained in an identical way as imaging element I with the exception that the antistress layer also comprised 140 mg/m² of a copolymer of styrene, methyl methacrylate and maleic acid as matting agent with a weight average diameter of 7 μm.

The imaging elements were placed in contact with a test target and exposed therethrough in a process-camera. In the next step each imaging element was immersed for 10 s at 24° C. in a freshly made developing solution having the following composition:

    ______________________________________                                         carboxymethylcellulose    4      g                                             sodium hydroxide          22.5   g                                             anhydrous sodium sulphite 120    g                                             hydroquinone              20     g                                             1-phenyl-4-methyl-3-pyrazolidinone                                                                       6      g                                             potassium bromide         0.75   g                                             anhydrous sodium thiosulphate                                                                            8      g                                             ethylene diamine tetraacetic acid tetrasodium salt                                                       2      g                                             demineralized water to make                                                                              1000   ml                                            pH (24° C.) = 13                                                        ______________________________________                                    

The initiated diffusion transfer was allowed to continue for s to form a silver image in the image receiving layer.

To remove the developed silver halide emulsion layer and the antistress layer from the hydrophilic base the developed monosheet DTR materials were rinsed for 5 s with a water jet at 40° C. in a LP 82 (tradename of a processor marketed by Agfa-Gevaert, Belgium).

Next, the imaged surfaces of the hydrophilic base were treated in a LP 82 (marketed by Agfa-Gevaert, N. V. of Belgium) for 15 s with a finisher at 45° C. to enhance the water-receptivity of the non-image areas and to make the image areas oleophilic ink-receptive. The finisher had the following composition:

    ______________________________________                                         Gebo (trade mark for a surfactant sold by                                                                250    mg                                            Chemische Fabrik Chem-Y, Gmbh, Germany)                                        polyethylene glycol 3000  75     g                                             potassium nitrate         12.5   g                                             citric acid               20.0   g                                             2-mercapto-5-heptyl-oxa-3,4-diazole                                                                      2.0    g                                             NaH.sub.2 PO.sub.4.2H.sub.2 O                                                                            20.0   g                                             5-bromo-5-nitro-1,3-dioxane                                                                              200    mg                                            sodium hydroxide          13.0   g                                             water to make             1000   ml                                            pH (20° C.) = 5.9                                                       ______________________________________                                    

The printing plates thus prepared were used for printing under identical conditions. The printing plates were mounted on the same offset printing machine (HEIDELBERG GTO-46). As fountain solution was used Rotamatic at 50% and as ink K+E 123 W from Kast and Ehinger, A. G., Germany. A compressible rubber blanket was used.

The quality of the printing plates was judged by the criteria of tendency for showing fingerprints (A) and change in photographic sensitivity by storage for 5 days at 57° C. and 34% R.H. of a package without interleaves, packed at 20° C. and 40% R.H. (B: a positive number means an increase in sensitivity). The results are given in table 1.

                  TABLE 1                                                          ______________________________________                                         Quality aspect  I        II                                                    ______________________________________                                         A               very bad good                                                  B               0.10 log It                                                                             0.03 log It                                           ______________________________________                                    

It is clear from these results that a printing plate obtained from an imaging element comprising an antistress layer having no matting agent (comparison element I) yielded an unacceptable bad result for the tendency to fingerprints. The change in photographic sensitivity on storage is also marked. A printing plate obtained from an imaging element comprising an antistress layer having a matting agent which has a weight average diameter of 7 μm (imaging elements II according to the invention) yielded a good result for the tendency to fingerprints. The change in photographic sensitivity on storage is also smaller. Furthermore the ink acceptance of imaging element II is clearly better than the ink acceptance of imaging element I. 

We claim:
 1. An imaging element comprising in the order given on a hydrophilic base (i) an image receiving layer containing physical development nuclei, (ii) a photosensitive layer containing a silver halide emulsion being in water permeable relationship with said image receiving layer and (iii) an antistress layer being in water permeable relationship with said image receiving layer and comprising gelatin, wherein said antistress layer comprises at least 30 mg/M² of a matting agent with a weight average diameter of at least 3 μm, which is a copolymer of styrene, methyl methacrylate and maleic acid.
 2. An imaging element according to claim 1 wherein said antistress layer comprises unhardened gelatin.
 3. An imaging element according to claim 1 wherein said matting agent has a weight average diameter of at least 4 μm.
 4. An imaging element according to claim 3 wherein said matting agent has a weight average diameter of at least 5 μm.
 5. An imaging element according to claim 1 wherein said antistress layer comprises between 50 and 500 mg/m² of said matting agent with a weight average diameter of at least 3 μm.
 6. An imaging element according to claim 5 wherein said antistress layer comprises between 100 and 200 mg/m² of said matting agent with a weight average diameter of at least 3 μm.
 7. An imaging element according to claim 1 wherein said antistress layer comprises gelatin or a gelatin admixture in an amount between 0.2 and 2 g/m².
 8. A method for making an offset printing plate according to the silver salt diffusion transfer process comprising the steps of:(a) image-wise exposing an imaging element according to claim 1, (b) applying an aqueous alkaline solution to the imaging element in the presence of (a) developing agent(s) and (a) silver halide solvent(s), (c) treating the imaging element to remove the layer(s) on top of said image receiving layer, thereby uncovering said silver image formed in said image receiving layer. 